1. Field of the Invention
This invention relates to an orthogonal transform coding apparatus for high-efficiency coding of picture signals.
2. Description of Prior Art
High-efficiency encoding has become important with the development of digital technology for use with picture signals. As an example of the high efficiency encoding technology, orthogonal transform coding has been known, in which time sequential signals to be inputted are transformed to orthogonal components such as, for example, frequency components. As the orthogonal transformation, a Fourier transformation, a discrete cosine transformation (hereinafter abbreviated as DCT) and a Hadamard transformation are well known. Especially, DCT has been considered as the orthogonal transform technology suitable for processing picture information.
An example of high efficiency coding will be explained below by referring to FIG. 29 showing an example of a conventional high efficiency encoding apparatus using a DCT. In FIG. 29, element 1 is an input terminal; element 2 is a blocking unit; element 3 is a DCT unit; element 4 is an adaptive quantizer; element 5 is a variable length encoder; element 6 is a data buffer, and element 7 is an output terminal.
A digital image signal inputted from the input terminal 1 is sent to the blocking unit 2 to be separated into blocks on a DCT unit basis. In the high efficiency encoding of image pictures, a two dimensional DCT for a block constructed of a total of 64 picture elements made of horizontal 8 pixels and vertical 8 pixels is often used. The thus blocked image signals are subjected to the two dimensional DCT by the DCT unit 3 to be transformed into DCT components. The thus obtained DCT components are quantized by the adaptive quantizer 4, and then subjected to variable length coding by the variable length encoder 5 and adjusted to a predetermined rate by the data buffer 6 and sent to the output terminal 7.
The variable length encoding is a way of coding in which a codeword of a higher generation probability is assigned a shorter code and a codeword of a lower generation probability is assigned a longer code. Table 1 shows the correspondence between three bit data 0, 1, 2, 7 and their variable length codes. In this example, numbers 0 and 1 are assigned a 2 bit code; numbers and 3 are assigned a three bit code, and numbers 4, 5, 6, and 7 are assigned a 4 bit code.
TABLE 1 ______________________________________ Data Variable Length Code ______________________________________ 0 0 0 1 0 1 2 1 0 0 3 1 0 1 4 1 1 0 0 5 1 1 0 1 6 1 1 1 0 7 1 1 1 1 ______________________________________
Since the DCT components usually show an exponential distribution, the general probability of 0 and 1 is considerably larger than that of 4, 5, 6 and 7, and the average number of bits after encoding is smaller than 3 bits. In this case, however, it is noted that when the variable length encoding is used, the data rate after encoding may change depending on the picture quality. As a result, in the conventional apparatus shown in FIG. 29, in order to prevent an overflow or underflow in the data buffer 6, the adaptive quantizer 4 increases the quantization width when the amount of data in the data buffer 6 is increased and decreases the quantization width when the data amount is decreased.
The conventional high efficiency coding apparatus using a DCT has the following problems:
(1) Since the variable length encoding is used, if an error bit occurs, code synchronization is disturbed, although it depends on the transmission line, and the decoding of data is prevented. Such a transmission error causes the picture quality to be greatly deteriorated. Therefore, it is difficult to employ the conventional DCT apparatus particularly in devices in which transmission errors can occur with a high probability such as in a VCR.
(2) In order to maintain the data rate at a constant value, a feed-back system using a buffer is conventionally used. However, actual image data is unbalanced and it is difficult to obtain the optimum coding by the feed-back technique. Particularly, when the amount of data of the front half of a picture image is small and that of the rear half is large, unnecessary data is assigned to the front half, and the amount of data for the rear half is reduced, resulting in a remarkable degradation in the picture quality.
In order to solve these problems of the conventional apparatus, applicants have invented and proposed an apparatus in which the amount of data after encoding is estimated before quantizing to determine an optimum quantization and the variable length encoding can be completely achieved in a small range (see U.S. Pat. No. 5,073,821, issued Dec. 17, 1991).